In tape automated bonding (TAB) microelectronic packaging, a set of beam leads (a "lead frame") electrically interconnects a semiconductor chip to a circuit-board substrate. The beam leads are supported on a polymer substrate called a TAB tape, and each frame of the TAB tape contains a new lead frame. Each lead frame contains an inner lead bond area where individual beam leads are aligned with a chip's contact location, and an outer lead bond area for electrically interconnecting the chip to a circuit board substrate. To make the desired interconnections between a chip and lead frame, a heated bonding head (thermode) forces the individual beam leads in the inner lead bond area against the chip's contacts to achieve a thermal, compression-bond.
TAB technology is moving in a direction of more leads and smaller spacings. Today, TAB lead sizes are on the order of two thousandth's of an inch widths on four thousandths centers. To assure proper registration, the semiconductor industry has been forced to use optical alignment techniques when placing chips on lead frames.
The art is replete with examples of automated article inspection systems and chip placement machines. In U.S. Pat. No. 4,759,073 to Shah et al, a wire bonder is disclosed wherein a chip is rotated four times, with each side being bonded sequentially. After each rotation, a signature area on the chip is determined and the chip is repositioned with respect to the signature area. In U.S. Pat. No. 4,738,025 to Arnold, an automated chip positioning system is described wherein both a chip and circuit board are visualized and viewing heads are calibrated to enable a positioning member to position the chip on the circuit board at a correct position and orientation.
While the viewing systems of Shah et al. and Arnold are relatively unsophisticated, a more elegant viewing system is described in U.S. Pat. No. 4,851,902 to Tesuka et al. This system is employed as a visual inspection unit to determine plating quality on a unit being inspected. An image is captured from reflected light off an IC frame. The image is used to correct errors in position between a reference stored image and itself. A significant problem exists with respect to reflected light vision systems in that they are highly susceptible to changes in reflectivity of the surface being examined.
A similar viewing system is described in U.S. Pat. No. 4,688,939 to Ray, but this system is directed to the inspection of solder bumps on a chip. A ring-light is placed above and about the chip so that its light is directed at an angle towards all sides of the chip, with the solder bumps thereby reflecting the light upwardly, while the remaining portions of the chip diffuse the light elsewhere. Here again, light reflectivity is a significant factor in differentiating the light reflected from the solder bumps from light reflected by the remaining surface areas of the chip.
Other imaging systems exhibit one or more of the following drawbacks: the resolution capability of the system is limited by the pixel (pel) size in the imaging electronics; the stored image against which a comparison is made is derived from the scanning of the "standard" device or substrate; the system examines the unit under test from a global point of view and is unable to specify with particularity, the precise details of an image which are inconsistent with the stored reference image; or the vision system is dependent upon analysis of binary image pels instead of using full grey-scale information. Prior art which exhibits one or more of those drawbacks is as follows: U.S. Pat. Nos. 4,481,664 to Linger et al., 4,500,202 to Smith; 4,555,798 to Broadbent, Jr.; 4,578,810 to Macfarlane et al.; 4,450,579 to Nakashima et al.; and 4,811,410 to Amir et al.
In order to implement a sophisticated chip/TAB lead frame imaging system, a chip placement/TAB tape movement machine must have many degrees of freedom to enable accurate positioning and bonding of chips to the lead frames. The design of such a machine requires that its movable elements be referenced to a datum to assure precise and reproducible movements. While the prior art describes the use of built-in calibration mechanisms, the weakness in such machines is that the datum surfaces are separate from other moving elements. Thus, if for any reason the calibration data is "off", the entire system becomes potentially inoperative. In a system with a plurality of moving elements, it is preferred to interrelate the reference datums among the elements, rather than with some independent, fixed structure.
Prior art references indicating various aspects of device imaging systems can be found in U.S. Pat. Nos. 4,236,306 to Hug et al; 4,507,605 to Gersel; 4,516,673 to Kashihara et al.; 4,606,117 to Takahashi et al.; 4,675,993 to Haroda; 4,677,258 to Kawashima et al.; 4,683,644 to Tange et al.; 4,803,358 to Kato et al; 4,813,588 to Srivastava et al.; 4,843,695 to Doe et al.; and in IBM Technical Disclosure Bulletin, Vol. 30, No. 3, August 1987, pp. 1197-1199 (Hutson) and Vol. 32, No. 11, April 1990, pp. 459,460 (Cipolla et al.).
Accordingly, it is an object of this invention to provide an improved method for aligning a contact pattern on a semiconductor chip with a TAB lead frame.
It is another object of this invention to provide an improved chip/TAB lead frame alignment method which is not limited in its accuracy by the pel resolution of an imaging system (i.e.: discrete size of individual sensors in an imaging system).
It is still another object of this invention to provide an improved method for aligning a chip with a TAB lead frame, wherein an imaging system used therewith is not dependent upon the reflectivity of the elements being joined.
It is yet another object of this invention to provide an improved chip placement machine whose movable elements have a plurality of freedoms of motion, with each movable element having a datum axis of motion against which the other elements are referenced.